From the days of the Roman aqueducts, to putting man in orbit, treatment of sewage has reduced human suffering through reduction of disease, and provided a more pleasant environment in high-density population areas such as cities and spacecraft. Municipal sewage treatment in particular has matured and is well characterized.
Typical municipal sewage treatment facilities as shown in FIG. 1 receive the raw sewage influent 10, primarily treat the raw sewage by one of several primary processes producing a primary sludge 11 and a settled effluent 12, digest the settled effluent in an aeration basin 13 via aerobic digestion, secondarily treat the aerobically digested stream by one of several secondary processes producing a biological sludge 14 and a treated water stream 15, then digest the primary and biological sludges (11 and 14, respectively) to produce a digested sludge 16. The digestion process provides an energy output in the form of methane and converts nitrogen bearing compounds, for example amine groups on amino compounds, to ammonia that is released into an aqueous phase. The digested sludge 16 is an input stream to the dewater facility 17 and contains about 3% by weight of solids. The dewater facility 17 exhausts two effluents, the first 18 having less than 1% solids, and the second 19 containing about 20% by weight of solids. The first low-solids dewater effluent 18 contains about 71/2 times the overall mass of the second solid-containing stream 19. Moreover, the first 18 low-solids dewater effluent contains a high concentration of ammonia, about 1000 ppm. The first low-solids dewater ammonia-containing effluent 18 is recycled back to the primary treatment portion of the waste water treatment plant, while the second solids-containing effluent 19 is generally prepared for landfill. The secondary treatment has a solids drain for sending solids to the dewater facility 17, and a main effluent 20 that may either be released to the environment or further processed before release to the environment.
One of the difficulties of the conventional sewage treatment plants is the ammonia in the first dewater effluent stream 18. At high concentrations (up to 1000 ppm ammonia), the second effluent stream 19 is mixed with incoming sewage influent 10, thereby significantly increasing the concentration of aqueous ammonia in the main sewage, settled effluent, and secondarily treated water flows. Aerobic digestion can be impeded because of the toxic effect of high ammonia concentration on the bacteria needed for the digestion. Hence, without further treatment beyond the secondary treatment, ammonia is released to the environment with subsequent algal growth and spoilage of natural waters.
Increased environmental concern precipitating recent legislation relating to disposal of sewage, including the Ocean Dumping Ban Act and the Clean Air Act, mandate that conventional sewage treatment methods will need to be replaced or modified to meet the new standards. Moreover, solutions are being sought to reduce nutrient loading in estuaries and inland waters.
According to U.S. Pat. No. 5,221,486 to Fassbender ('486), ammonia, in the presence of an oxidized nitrogen compound, for example a nitrate, may be removed from an aqueous stream by heating it from about 300.degree. C. to about 600.degree. C., and pressurizing it to maintain an aqueous or supercritical phase, then releasing nitrogen in the form of nitrogen gas and releasing a stream having an amount of ammonia concentration below 50 ppm. Further, in a paper entitled STORS and NitRem: Sludge-to-Oil Conversion and Nitrogen Removal for Waste Water Treatment Plants, presented at AWWA/WPCF Residuals Management Conference, Aug. 11, 1991, by AG Fassbender, it is shown in FIG. 1 of that paper that the second ammonia-containing stream from the dewater facility is sent through the process of '486 for removal of ammonia. It will be immediately recognized by one skilled in the art of sewage treatment that a tremendous amount of thermal energy would be required to heat that stream to a minimum of 300.degree. C. Thus, until the conception of the present invention, there still remained the problem of cost-effectively removing ammonia from sewage plant effluent.